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Interoperating with a Runtime Library

It is common to provide a set of run-time support functions that are written in another language than LLVM IR and it is trivially easy to interface to such a run-time library. The use of malloc and free in the examples in this document are examples of such use of externally defined run-time functions.

The advantages of a custom, non-IR run-time library function is that it can be optimized by hand to provide the best possible performance under certain criteria. Also a custom non-IR run-time library function can make explicit use of native instructions that are foreign to the LLVM infrastructure.

The advantages of IR run-time library functions is that they can be run through the optimizer and thereby also be inlined automatically.